Add a pixelRatio example

This example creates a screenshot, based on a certain pixelRatio.
```
Usage: phantomjs examples/pixelratio.js URL filename pixelRatio
```

`phantomjs examples/pixelratio.js https://www.github.com/ ~/Desktop/github-retina.png 2`
![github-retina](https://cloud.githubusercontent.com/assets/777823/5449948/64aa2bb6-84fe-11e4-9968-7318159d5655.png)

#10964